1. Field of the Invention
This invention relates to a luminance signal processing circuit for reducing noise in a liminance signal of a video signal, and being used in a video signal recording and reproducing apparatus such as a video tape recorder (VTR).
2. Description of the Prior Art
Conventionally, in the luminance signal reproducing circuit of VTR, a noise reducing circuit was provided in order to reduce the noise level in the reproduced luminance signal after frequency demodulation. FIG. 8 shows a block diagram of such conventional noise reducing circuit. In this diagram, the reproduced luminance signal enters into an input terminal 101, and its frequency components of, for example, 1 MHz or higher are taken out by a high-pass filter 102. Since the signal components are large in amplitude and noise components are relatively small in amplitude, the signal components of large amplitude are suppressed in an amplitude limiter 103 while only the noise components small in amplitude are left. These noise components are adjusted to be the same in amplitude as the input signal in a coefficient circuit 104 and supplied to a subtraction circuit 105 to be subtracted from the input reproduced luminance signal. Since visible noises are generally those of high frequency and small amplitude components of over 1 MHz, a signal reduced in the visible noise is obtained.
Another conventional example of the noise reducing circuit is shown in FIG. 10. In this example, the reproduced luminance signal entered into an input terminal 107 is fed to a 1H delay circuit 108 to be delayed for one horizontal period (1H), and is then fed into a subtraction circuit 109 to be subtracted from the input luminance signal. Incidentally, as it is well known, a video signal has a vertical correlation that two signal mutually spaced by 1H are extremely similar to each other, whereas such vertical correlation is not present in the noise. Therefore, the output of this subtraction circuit 109 is a luminance signal component having no vertical correlation with the noise. This signal is limited in amplitude by an amplitude limiter 110 which has an amplitude limit value approximately equal to the peak value of the noise, so that the luminance signal components of large amplitude having no vertical correlation are removed, and only the noise components are taken out. The noise components are substracted from the input signal in a substraction circuit 111. As a result, the input luminance signal is greatly reduced in the noise level, and is delivered to an output terminal 112.
It is also known, as shown in FIG. 11, to extract components having no vertical correlation by using a recursive comb filter, and subtract them from the input signal.
In FIG. 11, the video signal entered into an input terminal 1 passes through a recursive comb filter composed of adder 10, 1H delay circuit 11, and first coefficient circuit 12, so that the components having vertical correlation are extracted. This signal is fed into a second coefficient circuit 13, where its amplitude is adjusted to the input signal, and it is subtracted from the input video signal in a subtractor 14, so that the components having no vertical correlation are extracted, and an output signal is obtained through a third coefficient circuit 15. Supposing the coefficient of the first coefficient circuit 12 to be k (o&lt;k&lt;1) and that of the second coefficient circuit 13 to be 1-k, by selecting the coefficient of the third coefficient circuit 15 to be 1+k, the height of the peak of frequency characteristic of the output becomes constant, regardless of coefficient k.
The transfer function G in FIG. 11 is expressed as follows: ##EQU1## where .epsilon.=exp (-j.omega./f.sub.H)
From equation (1), if the frequency of the input signal is nf.sub.H (n being an integer), then .epsilon.=1, so G=0; if it is (n+0.5) f.sub.H, then .epsilon.=-1, so G=2. That is, the characteristic of the recursive comb filter is a typical comb filter profile, becoming bottom when the frequency of the input signal is nf.sub.H (n being an arbitrary integer, and f.sub.H a horizontal scanning frequency), and peak when it is (N+0.5)f.sub.H. When the coefficient k is small, the passing range is narrow, and when it is large, the range is wide.
There is still another noise reducing circuit as shown in FIG. 13. This is, for example, disclosed in the Japanese Unexamined Patent Publication No. 60-116290. In operation, an output of a 1H delay circuit 126 and an input signal are added in an adding circuit 127. The sum is passed through a high-pass filter 128 and a coefficient circuit 129, and is added with an input signal in an adding circuit 125. This sum is the input to the 1H delay circuit 126 of a recursive comb filter. By using this filter and an equalizer circuit 130 which flattens the frequency characteristic of its output, the circuit is operated to work as a comb filter of a wide passing range when the frequency of input signal is low or as a comb filter of a narrow passing band when the frequency of input signal is high, so that the noise components in the high range can be effectively lowered. Furthermore, in a subtraction circuit 132, a difference signal of this output signal and the input signal is made, and a signal of large amplitude is taken out in a clip circuit 133, that is, a signal of high level and having no vertical correlation is taken out, and added with the output of the equalizer circuit 130 is an adding circuit 134 thereby obtaining a signal without vertical correlation. Thus, it is intended to greatly reduce the visible noise components in the high range if there is vertical correlation, and not to lower the vertical resolution if the frequency is low or there is no correlation.
However, in the conventional noise reducing circuit shown in FIG. 8, if the reproduced luminance signal contains an edge as shown in FIG. 9 (a), the output of the high-pass filter 102 becomes as shown in FIG. 9 (b) while the output of the limiter 103 becomes as in FIG. 9 (c), so that the noise components are eliminated in the amplitude limited area. As a result, the noise is left over in the edge portion as shown in FIG. 9 (d).
In the circuit in FIG. 10, if there is a vertical correlation in the luminance signal, the noise of the edge portion may be improved as compared with the circuit in FIG. 9. However, the improvement of S/N ratio of the entire signal is only several dB. Further, since this has a comb filter characteristic to pass the frequency of an integer times of horizontal scanning frequency f.sub.H by a similar degree over the entire frequency region, the vertical resolution is lowered, and the noise is particularly visible in the low frequency region.
In the recursive comb filter type in FIG. 11, when the passing range is widened, more noise components may be decreased. But since the signal components are decreased too, the vertical resolution is lowered. It has been hence considered to strengthen the decrease of noise components from 1 MHz to 2 MHz that are particularly visible. That is, the coefficient k of the recursive comb filter is raised at the frequency desired to further decrease the noise, or it is sufficient to make the coefficient k larger when the frequency is higher, and smaller when it is lower. For example, a band-pass filter is added to the coefficient circuit in the former case, or a high-pass filter is used in the latter case.
The following explanation refers to the high-pass filter, but the same applies to the band-pass filter.
However, in the circuit in FIG. 11, since there are three coefficient circuits and their coefficients are all different, it is difficult in the circuit design to vary the values of each k depending on the frequency of input signal, and if it were possible, the filter characteristic would be changed due to discrepancy among circuit elements.
The circuit in FIG. 13 solves the above-discussed problems to a certain degree, in which the vertical resolution is not lowered in the low range and the noise is effectively reduced in the high range. But to maintain the frequency characteristic, two filter circuits using resistor and capacitor are needed, such as the high-pass filter and equalizer circuit. Fluctuation of characteristics of the two filters may affect the entire characteristic, or when varying the frequency characteristics, the two filters must be varied, which was not so convenient.